1. Field of the Invention
The present invention relates to a peer to peer (P2P) type communication system including a plurality of node devices mutually communicable through a network.
2. Discussion of the Related Art
As this kind of peer to peer type communication system, a distributed content storing system where content data are distributed and located (distributed storing) into a plurality of node devices is known. By using this system, fault tolerance and property of distributing accesses are enhanced. Location of content data thus distributed and stored can be efficiently searched in use of a distributed hash table (hereinafter referred to as “DHT”) as shown in for example in Japanese Unexamined Patent Publication No. 2006-197400. By this nodes devices respectively establish session with a node device storing content data, related to the above-mentioned search, and thereafter enjoy service of the content data from the node device.
These node devices are connected to a network through a router as a rely device and in communication between node devices, for example, TCP (sending Control Protocol)/IP (Internet Protocol) is used. Each of the node devices prepare (open) a port for communication with other node devices. When session (i.e. establish a communication path) is established, a two-way communication through the router and network is enabled.
Meanwhile, NAT (Network Address Translation) is known as an art that node devices without a global IP net address, allocated to it, and with only a private IP address, allocated to it, are connected to the network. In this NAT, an NAT compatible router mutually exchanges a private IP address and a global IP address, and therefore node devices with only private IP address allocated to it can transparently connect to a network.